John\Pam,
     
          As an aside, one other concern I could forsee is the possibility 
of fracturing solder joints.  This of course would be dependent on quite a 
few variables such as board size/thickness, how badly warped/twisted, 
assembly process(thru-hole/SMT, mixed technology), how flat the board is 
during wave/reflow & what type of application the board is being used in 
(i.e put in a card rack, torqued down in a case, etc.).  We have run tests 
comparing bow&twist measurements on boards by serial number both as raw 
boards and also after assembly/wave.  Some boards actually improved and some 
got worse, why? I'm not sure, maybe due to which way it was bowed/twisted 
before it hit wave.  Any way I thought you might want to take this info. 
into consideration on your mission.

Eric Dovenbarger
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 ----------
From: TechNet-request
To: jol; TechNet
Subject: Re: Bow and Twist
Date: Thursday, May 09, 1996 9:49AM


     John,

As a packaging engineer, this is my advice on your situation.  It does not
directly answer your question on how to interpret the spec, but provides
insight
as to the intent of the requirement and how I've handled similar problems
with
much larger boards.

        1) As a rule of thumb, I typically have warp and twist measured on
the
boards in their application orientation.  Typically when a board is laid
flat,
especially if it is populated, gravity will remove some if not all of the
warp
and twist.  If the customer is using the board is this orientation, then it
may
be acceptable depending on spacing and other mating components including
packaging restraints.  If the board is going into a card cage then warp and
twist can cause definite problems.  These problems can range from arcing
between
boards to the boards physically jumping the card guides.  As an end user, it
is
very difficult to define and measure the amount of warp and twist in a
populated
board.  In the past, I've taken measurements on boards vertically on edge
from a
known flat surface at a known distance, e.g. a rigid metal plate.

        2) If the end user is having problems with the boards in their
system,
yes it is a concern.  Depending on the environment that your customer is
subjecting the boards to, the warp and twist can cause failures during
testing.
You'll need to work with your supplier and explain to them the concerns of
your
customer and the dissatisfaction in their quality and that the product does
not
meet its intended use.

I hope these thoughts from my experiences prove helpful in resolving your
dilemma.

Pam Morrison
Lockheed Martin
Information Systems

______________________________ Reply Separator
_________________________________
Subject: Bow and Twist
Author:  [log in to unmask] at ORL-SMTP-G
Date:    5/8/96 12:17 PM


A question on interpretation of the IPC method IPC-TM-650, 2.4.22.

I have a batch of boards 400mm square, which are quite clearly twisted when
held free in my hands. When I lay a board horizontal, it straightens under
its own weight. All corners touch the plane and I am left with a small
residual bow of 1 to 2mm, say 0.25 to 0.5%.

My supplier says it is in specification, but my customer (who cannot carry
out the prescribed test on the populated board) comments on the degree of
twist.


Questions:

1) Is this the correct interpretation of the test and specification?

2) Should I be concerned?

Regards

John Loveluck
Tadpole Technology
Cambridge
United Kingdom
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